1. Field of the Invention
Embodiments of the present invention relate generally to magnetic storage systems and methods and, in specific embodiments, to a magnetic storage system including a head and a servo controller in which the servo controller sets a threshold, such as a write fault window (WFW), a PES plus Velocity (PPV) threshold, and the like, to a first value based on at least one of an on-track track misregistration (TMR) value and a seek settle TMR value, and sets the threshold to a second value that is different from the first value when it is determined that the head is subject to a vibration condition.
2. Related Art
Magnetic storage systems, such as disk drives, are widely used in computers and other electronic devices for the storage and retrieval of data. Important design considerations for disk drive manufacturers generally include: (a) data storage capacity; (b) data transfer rate; (c) data integrity and reliability; and (d) manufacturing cost. Of those considerations, data integrity and disk drive reliability are of particular importance, because incorrect or corrupted data may lead to erroneous calculation results, incorrect program execution, or other similar problems.
In general, related art disk drives comprise one or more disks for storing data, an actuator, and one or more transducers or heads. Each head is operable to read data from and write data to concentric circular tracks on a surface of a corresponding disk. The heads are typically attached to the actuator, and when a head performs a read or a write operation, the actuator is moved so that the head is positioned over a center of a selected track for the operation.
In recent years, disk drive manufacturers have sought to increase the data storage capacity of disk drives while controlling the manufacturing cost. One solution has been to increase track density by increasing the number of tracks per inch (TPI) on each disk. As TPI has increased, tracks have become narrower, and maintaining data integrity has become a greater design challenge because data errors can occur with smaller amounts of movement of a head away from a track center during a read or a write operation.
Movement a head away from a track center can lead to an off-track read or an off-track write. An off-track read occurs when a head is positioned over a wrong track during a read operation and the head reads data from the wrong track. In such an instance, the incorrect data would have to be discarded, the head repositioned over the correct track, and the head would then have to read in the correct data. As a consequence, the data transfer rate of the disk drive would be reduced, because the time spent reading the wrong data would be wasted. Even worse than an off-track read is an off-track write. An off-track write occurs when a head is positioned over a wrong track during a write operation and the head writes data to the wrong track. As a result of an off-track write, data integrity is adversely affected, because existing data on the wrong track is improperly overwritten and is potentially lost.
Thus, to prevent data errors, it is preferable to maintain a head over a center of a selected track during a read or a write operation. In order to position a head during a read or a write operation, prior art disk drives typically comprise a servo controller and have embedded servo sectors located in the tracks of each disk. The embedded servo sectors are located between data sectors and contain predetermined patterns from which a position of a head during an operation can be determined.
During read and write operations to a selected track, a head reads data from embedded servo sectors of the selected track and provides the data read from the embedded servo sectors as servo information to a servo controller. The servo controller receives the servo information provided by the head and determines a position error signal (PES) from the servo information. The PES is a signal that is indicative of a position of the head relative to a center of the selected track. The PES is usually specified in terms of a percentage (+/−) that the head is away from the center of the selected track. The servo controller then sends a positioning signal to reposition an actuator based on the PES so that the head is moved toward the center of the selected track.
Using a PES to reposition a head of a disk drive over a track center generally permits for satisfactory performance when the disk drive operates under normal, vibration free, conditions. Vibration free conditions exist when a disk drive is not subject to movement or vibrations from external sources. However, even under vibration free conditions, there may still be some internal factors that can cause misalignment of a head. A non-exhaustive list of such internal factors is discussed in U.S. Pat. No. 6,094,806 entitled “Method for Fabricating a Dual Element Head”, and includes spindle run out, resonances and disk flutter, thermal track shift, head settling, actuator interactions, improper servo writing, and the like.
Expected movements of a head away from a track center under vibration free conditions can be statistically or experimentally determined, and can be quantified as values, such as an on-track track misregistration (TMR) value and a seek settle track misregistration (TMR) value. An on-track TMR value is a percentage value representing a maximum amount of misalignment of a head from a track center that is probable during normal track following operations. Similarly, a seek settle TMR value is a percentage value representing a maximum amount of misalignment of a head from a track center that is probable during seek settle under vibration free conditions. Seek settle occurs when an actuator moves a head to a new track for a read or a write operation. When moving the head to the new track, the actuator may overshoot the track and have to be repositioned. Seek settle refers to the time during which the head settles into the new track before beginning the operation. The on-track TMR value and the seek settle TMR value for a particular head of a particular disk drive may be statistically approximated from a known operation of similar disk drives, or may be determined during a self-test operation of the particular disk drive.
When operating in various environments, a disk drive may be subject to external forces in the form of vibrations or shocks. Vibrations may be caused by, for example, mechanical interactions between disk drives that operate on a same computer rack, and the like. When a disk drive operates under vibration conditions, an actuator on which a head is located may be caused to oscillate and, thus, the head may move farther distances away from a track center. Disk drives are typically designed to be able to handle a certain level of vibration so that they are still able to operate reliably under some vibration conditions.
A shock event differs from vibrations in that a shock is typically a one time external force that acts on a disk drive and causes a head to move far away from a track center. A shock may be caused by, for example, a strike on a disk drive, dropping a laptop in which a disk drive is located, and the like. When a shock occurs during a write operation, a head will typically be forced off-track, and there is a potential for an off-track write if the write operation is not disabled quickly.
In order to disable a write operation when a head is moved a certain distance away from a track center, a servo controller is typically designed to monitor a PES, and if the PES exceeds a write fault window (WFW), then the servo controller provides a signal to inhibit the write operation. The WFW, also known as an off-track threshold or write unsafe (WUS) limit, specifies a percentage (+/−) that if exceeded by a PES will cause a servo controller to provide a signal to disable a write operation.
The WFW has been traditionally set to a fixed value for all heads of all disk drives of a single family that are manufactured at a same time. One approach has been to experimentally determine an expected worst case off-track capability (OTC) of heads in the disk drives, where the worst case OTC represents a maximum off-track position at which the heads may still operate reliably. Then, the WFW would be set for all drives in the family based on the expected worst case OTC. The WFW in such an approach may be set to allow a write operation to continue during a vibration event that does not cause a head to move beyond a position specified by the worst case OTC.
Another approach to setting the WFW has been to determine an expected narrowest width of read elements of heads for all disk drives in a disk drive family. Then, the WFW for all disk drives in the family would be set based on the narrowest expected width of the read elements of the heads. For example, the WFW for all disk drives would be set to 50% of the width of the narrowest expected read element in order to prevent errors, such as sliver errors where old data remains and is read even when new data has been written to a track.
Recently, the related art has considered the possibility of setting the WFW for individual heads of individual disk drives instead of using a single WFW for all heads of all disk drives in an entire family of disk drives. Also, the related art has considered expanding the WFW from a default value. Such related art includes (1) U.S. Pat. No. 6,717,757 entitled “Variable Write Fault Protection Window”; (2) U.S. Pat. No. 6,714,372 entitled “Method of Manufacturing a Disk Drive by Measuring the Read Width in the Servo-Writer to Set the Write Unsafe Limit”; and (3) U.S. Pat. No. 6,795,262 entitled “Method of Modifying On-Track Declaration Algorithm of a Disk Drive Based Upon Detection of External Vibration Condition”.
If a WFW is set to maximize disk drive capability based on a worst case OTC or based on a width of a narrowest expected read element of a head, then writing data when the WFW is exceeded by a PES may result in an off-track write or a potential sliver error because the head would have exceeded its capability. Such a setting of the WFW would be effective in preventing data errors if the PES were continuous. However, when using embedded servo sectors, the PES is not continuous. Rather, the PES consists of a number of discrete sample points that are generated when a head reads servo information from embedded servo sectors as a disk spins.
FIG. 1 is a graph illustrating how data integrity may be compromised if a WFW is set as in the related art. The curve in the graph of FIG. 1 represents a PES for a head with sample points P1-P9 at sample times T1-T9. The sample points P1-P6 reflect variations in the PES that may occur during normal track following operations due to, for example, internal factors as discussed above. At point P7, the PES reflects that the head is moving farther away from track center. The movement may be due to, for example, a shock to the disk drive. At time T8, the PES value P8 reflects that the head has moved father away from track center, but writing is not yet disabled, because P8 is still less than the WFW. Finally, at time T9, the PES value P9 has exceeded the WFW, so writing is disabled.
A problem with the related art can be seen in the example. As shown in the example, the PES would exceed the WFW sometime between time T8 and time T9, but writing is not disabled until time T9, which creates the possibility for an off-track write. The delay in disabling writing occurs because the value of the PES is only known when samples are obtained during the time that the head reads servo information from the embedded servo sectors. When the head is writing data to data sectors, the head is said to be “flying blind” because no servo information is generated to correct a position of the head. In the example of FIG. 1, from the time after T8 when the PES would exceed the WFW to the time T9 when writing is disabled, the head may perform an off-track write or create the potential for a sliver error, because the WFW in the related art represents an expected limit of the capability of the head. As a result, data integrity may be compromised due to an off-track write or a potential sliver error when writing continues even when the head has moved to a position beyond the WFW.
One solution that has been proposed to detect an off-track write sooner is to increase a number of embedded servo sectors in each track. Increasing the number of embedded servo sectors in a track leads to an increase in a frequency at which sample are obtained for a PES and, thus, may allow for an off-track write to be detected sooner. However, increasing the number of embedded servo sectors is usually not desirable, because increasing the number of embedded servo sectors leaves less room on the disk for data sectors and, as a result, data storage capacity is reduced. Also, a higher sampling rate of a PES demands faster computation power, which may lead to an increase in manufacturing cost.
Another solution that has been proposed to detect an off-track write sooner is to include a hardware shock sensor in the disk drive. In such a configuration, when a signal from the hardware shock sensor exceeds a shock sensor threshold, writing is inhibited. The hardware shock sensor typically detects a shock event sooner than the shock can be detected using a PES and, thus, can reduce a probability of off-track write. However, including a hardware shock sensor in a disk drive has the disadvantage that manufacturing cost is increased due to a cost of the hardware shock sensor.
A further solution that has been proposed to detect an off-track write sooner is to calculate a predicted PES (P+V) from a known PES (P) and a velocity (V) and to compare the predicted PES with a PES Plus Velocity (PPV) threshold. A predicted PES may be calculated in different ways, and one way to calculate a predicted PES is disclosed in U.S. Pat. No. 6,496,315 entitled “Disk Drive with Off-Track Write Prevention”, where a PES (Pn) and velocity (Vn=Pn−Pn−1) are calculated by reading data at an nth servo position, and a predicted PES is calculated as: predicted PESn+1=Pn+Vn. A servo controller calculates the predicted PES and sends a signal to inhibit writing when the predicted PES exceeds the PPV threshold. Other methods of calculating a predicted PES are known in the art. The PPV threshold in the related art has typically been set to a fixed value that accounts for a worst case disturbance during which a disk drive may still operate reliably. Calculating a predicted PES requires more computation power and, thus, may increase the manufacturing cost of a disk drive.
In light of the above mentioned problems, there is a need to improve data integrity and disk drive reliability when a disk drive is subject to external forces, such as a shock. Also, there is a need to improve data integrity without decreasing a data storage capacity or increasing a manufacturing cost. Furthermore, there is a need to improve data integrity while not having a large detrimental impact on a data transfer rate.